Intake manifold for automotive fuel injection system

ABSTRACT

An intake manifold for fuel injection system of an internal combustion engine having a main throttle valve, and an auxiliary throttle valve each communicating with the atmosphere. The suction inertia effect (ram effect) of the intake manifold contributes to the improvement of the engine torque by operating the auxiliary throttle valve in relation to the opening of the main throttle valve and engine speed.

United States Patent Inventor Shuya Nambu Yokohama, Japan Appl. No. Filed Patented Assignee Aug. 3, 1969 Aug. 24, 1971 Nissan Motor Company, Limited Yokohama, Japan INTAKE MANIFOLD FOR AUTOMOTIVE FUEL INJECTION SYSTEM 8 Claims, 16 Drawing Figs.

U.S.Cl 123/139 BG,

123/32, 123/52 M, 123/119 D, 123/124, 123/139 AW, 123/140 Int. Cl F02m 39/00, F02b 3/00, F02m 23/04 [50] Field of Search ..123/140,32, 139.17, 52 M,52 MV;123/119 D, 124, 139.18

[56] References Cited UNlTED STATES PATENTS 2,033,211 3/1936 Tice 123/139.17

Primary Examinen-Wendell E. Burns AttomeysRobert E. Burns and Emmanuel J. Lobato ABSTRACT: An intake manifold for fuel injection system of an internal combustion engine having a main throttle valve, and an auxiliary throttle valve each communicating with the atmosphere. The suction inertia effect (ram effect) of the intake manifold contributes to the improvement of the engine torque by operating the auxiliary throttle valve in relation to the opening of the main throttle valve and engine speed.

PATENIED AUB24 I97! ENGINE TORQUE ENGINE TORQUE SHEET 2 UF 7 ENGINE SPEED ENGINE SPEED PATENIEU M1824 Ian SHEET 3 0F 7 PATENTEUIAUBZMHYI 3.601.108

' sum 5 OF 7 FULL OPEN AUX THROTTLE VALVE OPENING n n2 ENGI NE SPEED PATENIEU AUB24I9TI 3,601. 108

sum 6 0r 7 FULL OPEN AUX THROTTLE VALVE OPENING n ENGINE SPEED PATENIEII MIG24I97I $601,108

SHEET 7 0F 7 hi4 F/g. L5 5 INTAKE INTAKE vAI vE I \/E E OPEN EN 882' 52% l p MA I '7 O O V V V TIIvIE 1 1 I O: I 3 T4 a 4 #2 l V V TD.C. TOO. TD-C. TOO. TOO. TDC

F/g. L36 HQ L30 INTAKE INTAKE vAI vE VALVE E OPEN OPEN OLIJ p I 7 s T a: g n AAA/IN, O n I A A 85 I V b I v g TIME I E a O "In 0 TDC. TDCI TDC. TDC. TDC UM;

INTAKE MANIFOLD FOR AUTOMOTIVE FUEL INJECTION SYSTEM This invention relates to an intake manifold for fuel injection system of an internal combustion engine more particularly to such manifold employing in addition to a main throttle valve an auxiliary throttle valve.

There generally exist two types of intake manifold for fuel injection system of an internal combustion engine; one singlethrottle valve type and the other plural-throttle valve type which has a plurality of throttle valves. In practice, the singlethrottle valve type is mostly utilized because it is less expensive, easy to operate its accelerator pedal and also easy to meter the amount of intake air.

In the conventional air intake manifold for fuel injection system of an internal combustion engine, an air chamber is provided posteriorly of the throttle valve so that the air is introduced through individual air branches or passages to the respective cylinders or combustion chambers of the engine so as to reduce the suction resistance of the intake air and also to improve the performance of the engine by utilizing the suction inertia of the air.

In such a conventional intake manifold for fuel injection system such as, for example, the intake manifold for fourcycle four-cylinder engine in which the firing order is from the first, then third, fourth to second cylinders, the air intake operation is achieved in this order at the interval of crank angle of 180 so that there exists overlap in air intaking operations between the first and third, between the third and fourth, between the fourth and second, and between the second and first cylinders excepting between the first and fourth, and between the second and third cylinders.

Though the interference of intake air of the adjacent cylinders does not occur even if the period of air intaking operations between the adjacent cylinders is overlapped in case the volume of the air chamber is large enough, it is impossible to provide such a large air chamber in the actual engine with the result that the interference of intake air of the adjacent cylinders cannot be avoided because the normal volume of the conventional air chamber is approximately half that of the engine cylinder volume.

This interference of the intake air of the adjacent cylinders introduces at low engine speed maximum torque in the engine operation due to the effect of the apparent suction inertia and at intermediate and high speeds a lower torque due to the disturbance of the true suction inertia corresponding to the length of the air branches.

This invention contemplates eliminating the aforementioned disadvantages of the conventional intake manifold for fuel injection system and provides an air intake manifold for fuel injection system of an internal combustion engine which has in addition to the main throttle valve an auxiliary throttle valve communicating with the atmosphere for removing the suction interference, and in which the suction inertia effect of the intake manifold contributes to the improvement of the engine torque by operating the auxiliary throttle valve in relation to the opening of the main throttle valve and engine speed.

The features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an arrangement of intake manifold with its related parts as one example of the present invention;

FIGS. 2, 3 and 4 are graphical representations of the relationship between the engine speed and torque when the intake manifold of the present invention is used;

FIG. 5 is a plan view of the intake manifold of another example ofthe present invention;

FIG. 6 is a sectional view of the intake manifold taken along the line Vl-VI shown in FIG. 5;

FIG. 7 is a schematic view of a control device for mechanically opening and closing the auxiliary throttle valve in accordance with the present invention;

FIG. 8 is a schematic view of another control device for mechanically opening and closing the auxiliary throttle valve according to the present invention;

FIG. 9 is an operational graph showing the opening of the auxiliary throttle valve in relation to the engine speed shown in FIG. 8;

FIG. 10 is a schematic view of an electric control device for opening and closing the auxiliary throttle valve in accordance with the present invention;

FIG. 11 is a schematic view of another example of an electric control device shown in FIG. 10;

FIG. 12 is an operational graph showing the opening of the auxiliary throttle valve in relation to the engine speed shown in FIG. 1 1; I

FIGS. 13A and 13B are graphs showing the relationship between the air pressure wave forms and time of the conventional air intake manifold; and I FIGS. 13C and 13D are graphs showing the relationship between the air pressure wave forms and time of the air intake manifold constructed in accordance with the present invention.

In FIG. 1, the arrangement of the intake manifold with its related parts as one embodiment of the present invention for an internal combustion engine 10 having a fuel injection pump 11 driven at half of engine speed for injecting the fuel from a plurality of nozzles 12 into an intake hole 13 of a cylinder 14 by being controlled or measured in response to the opening of a main throttle valve 15 for controlling the amount of intake air and the engine speed, is shown as comprising an air cleaner 16 for admitting and filtering atmospheric air, a throttle valve chamber 17 connected with the air cleaner l6 and having the throttle valve 15 connected with an accelerator pedal 18, an air chamber 19 connected with the throttle valve chamber 17 at one end, and a plurality of individual branches or passages 20, 21, 22 and 23 each connecting the air chamber 19 with a cylinder 14 of the engine 10. The intake manifold further comprises an auxiliary throttle valve chamber 24 connected with the other end thereof and having an auxiliary throttle valve 25, and an air cleaner 26 connected with the auxiliary throttle valve chamber 24 for venting admitting and filtering atmospheric air to be fed to the air chamber 19 through the auxiliary throttle valve chamber 24. In this arrangement thus constructed, the air branches 20, 21, 22 and 23 are straight so that suction inertia, that is, the action that the weight of the intake air tends to be accelerated by the intake vacuum in the cylinder in the air branches and then tends to enter the intake valve abruptly, is provided, and the air branches 20, 21, 22 and 23 are connected anteriorly thereof with the air chamber 19 connected with the throttle valve chamber 17 having the throttle valve 15, and are connected posteriorly thereof with the intakeport for a suction valve (not shown).

In consideration again of the conventional arrangement of the air intake manifold with reference to FIG. 13A, the air chamber is common to the respective cylinder for supplying the suction inertia and cannot actually be enlarged to a sufficient size with the result that there occurs pressure fluctuation therein so that'this fluctuation is added to the suction inertia to cause the apparent inertia to be increased while this apparent inertia is reduced at the desired engine speed. It means that as seen in FIG. 13A, since the pressure p of the intake port when the intake valve of the first cylinder closes at low engine speed corresponds with the crest of the inner pressure q within the air chamber produced due to the effect of the suction interference of the third cylinder adjacent to the first cylinder in the firing order, the pressure p increases at low engine speed. On the other hand, as shown in FIG. 138 at intermediate and high engine speed since the time when the intake valve closes corresponds with the valley of the pressure q, the pressure p is decreased so much that as illustrated by the solid line in the graph shown in FIG. 2 the torque is lowered due to the reduction of the volumetric efficiency, because the pressure wave form is delayed with respect to the time of the opening of intake valve with increasing engine speed due to the inertia of the air. In other words, the intake valve opens earlier with respect to the pressure wave form with the result that a negative effect occurs. However, the intake manifoldof the present invention comprises the auxiliary throttle valve chamber 24 connected with the other end thereof with respect to the main throttle valve chamber 17 and having the auxiliary throttle valve 25, and the air cleaner 26 connected with the auxiliary throttle valve chamber 23 for admitting and filtering the atmospheric air to be fed to the air chamber 19 through the auxiliary throttle valve chamber 14 wherein the engine performance is improved by the control of the auxiliary throttle valve 15in accordance with the invention.

If the auxiliary throttle valve 25 is substantially closed during the full load or substantially full load operation of the engine, the torque of the engine is as shown by a solid line in F IG. 2, as previously described the maximum torque is produced in the engine due to the positive effect of the suction interference at low engine speed while at intermediate and high engine speed the torque of the engine decreases due to the negative effect of the suction interference. However, if the auxiliary throttle valve 25 is fully opened so that the air chamber 19 communicates with the atmosphere through the auxiliary throttle valve 25 in the auxiliary throttle valve chamber 24 and air cleaner 26, as seen in FIGS. 13C and 13D the pressure within the air chamber does not substantially change and accordingly the suction interference is removed with the result that though the torque of the engine decreases at low engine speed, the true suction inertia is effected to a maximum extent so that the torque of the-engine becomes as shown by a broken line in FIG. 2 and accordingly the torque of the engine is improved at intermediate and high engine speed.

Referring now to FIG. 3, which shows a graphical representation of the relationship between the torque and the engine speed in case of using the intake manifold of the present invention, the engine speed when the torque of the curve 27 is maximum as the auxiliary throttle valve 25 is fully closed is designated by 11 and the engine speed when the torque of the curve 28 is maximum as the auxiliary throttle valve 25 is fully open is designated by n, and if the auxiliary throttle valve 25 is gradually opened from fully closed state to fully open state between the engine speeds n, and n the combination of the suction interference and the suction inertia effects introduces the torque curve designated by 29 in FIG. 3. Accordingly, if the auxiliary throttle valve 25 is fully closed until the engine speed becomes It, then the auxiliary throttle valve 25 is gradually opened from fully closed to fully opened with the engine speed being increased from n to n and then the auxiliary throttle valve 25 is kept fully opened, the torque curve is improved by the shaded area designated by 30.

In this case, if the air branches 20, 21, 22 and 23 are shortened and the torque characteristics of the engine when the auxiliary throttle valve 25 is fully opened is of high-speed type,

the torque and maximum power of the engine is further improved in intermediate and high speeds.

Referring now to FIG. 4, which shows graphical representation of the relationship between the torque and the engine speed in case of using the intake manifold of the present invention, if the engine speed corresponding to the cross point between the torque curve 31 when the auxiliary throttle valve 25 is fully closed and the torque curve 32 when the auxiliary throttle valve 25 is fully opened is designated by n and the auxiliary throttle valve 25 is abruptly opened to full position from fully closed position at this engine speed n, the engine torque becomes as designated by the torque curve 31 when the auxiliary throttle valve 25 is fully closed until the engine speed n and the engine torque becomes as designated by the torque curve 32 when the auxiliary throttle valve 25 is fully opened over the engine speed n, the engine torque is improved by the area designated by 33.

In this case, if the manifold branches 20, 21, 22 and 23 are shortened and the torque characteristics of the engine when the auxiliary throttle valve 25 is fully opened is of intermediate speed type, the torque of the intermediate speed is further improved.

Referring now to FIGS. 5 and 6, which show another embodiment of the intake manifold of the present invention, the auxiliary throttle valve chamber 25 may be mounted on an intermediate portion of the intake manifold 19.

In the embodiment of FIGS. 5 and 6, the locations of the main throttle and the auxiliary throttle may be interchanged, the auxiliary throttle being located at one end of the intake manifold and the main throttle being at an intermediate position.

The embodiment shown in FIGS. 5 and 6 may cause the same effect as the first embodiment shown in FIG. 1.

The operation of the auxiliary throttle valve 25 may be mechanically or electrically controlled by detecting the engine speed by means of a centrifugal governor, the amount of the suction air, the voltage generated by the generator, lubrication oil pressure or coolant pressure when the main throttle valve 15 opens over a predetermined degree. Two embodiments of the control of the auxiliary throttle valve 25 will hereinafter be described.

Reference is now made to FIG. 7, which shows a control device for mechanically opening and closing the auxiliary throttle valve in accordance with the present invention. The control device comprises a link 18a pivotally connected at one end with the accelerator 18, an angled lever 18b fixedly connected with the throttle valve and having one arm pivotally connected with the other end of the link 18a, and another link 18c pivotally connected with the other arm of the angled lever 18b, a lever 18d fixedly connectedwith the auxiliary throttle valve 25 and pivotally connected with the other end of the other link 18c.

Referring now to FIG. 8, which shows a control device for mechanically opening and closing the auxiliary throttle valve in accordance with the present invention, the mechanical control device 40 comprises an angled lever 41 pivotally connected to the vehicle body 42 and having two arms 43 and 44. One arm 43 is connected with the main throttle valve 15 by a link 43a to cause the one arm 43 to be interlocked with the main throttle valve 15. A first lever 45 is pivotally connected with the vehicle body 42 at one end and is connected at an intermediate point with the other arm 44 of the angled lever 41 by a link 44a to cause the lever 45 to be interlocked with the angled lever 41. The lever 45 also has a roll support 46 rotatable mounted at the other end thereof. A centrifugal governor 47 driven by the engine has a plurality of weights 48, a plurality of angled levers 49 each connected at one end with the weight 48, a first rod 50 having a disc 51 fixed at one end thereof and engaged at one surface with the other ends of the angled levers 49 and inserted at the other end through a first hole 52 formed in the vehicle body 42. A spring 53 is so disposed between the disc 51 of the rod 50 and the vehicle body as to press the disc 51 against the centrifugal force of the weight applied to the other surface of the disc from the surface applied by the tension of the spring 53 supported on the vehicle body 42. A second lever 54 supported at an intermediate point by the vehicle body 42 is interlocked with the auxiliary throttle valve 25 at one end by a link 54a. A second rod is slidably inserted into a second hole 56 formed in the vehicle body 42 and is connected at one end with the other end of the lever 54. A third lever 57 is supported at an intermediate position by the roll support 46 rotatably mounted on the first lever 45 and is connected at one end with the other end of the rod 55 and with a second spring 58 which is fixed at one end to the vehicle body 42 for pressing the lever 57 rightwardly in the drawing. The other end of the lever 57 is engaged with the end of the rod 50 of the governor. The third lever 57 is formed with a cam recess 570 as to move the auxiliary throttle valve 25 in response to the movement of the main throttle valve 15 and engine speed. A stop 59 is provided for limiting the movement of the third lever 57.

In operation of the device thus constructed, the main throttle valve 15 is interlocked with the lever 45 through the angled lever 41 so that when the main throttle valve 15 is fully opened or substantially opened the lever 57 is contacting with the roller 46 of the lever 45 but when the main throttle valve is not fully opened nor substantially opened the lever 57 is not contacting with the roller 46. If the main throttle valve 15 is fully opened or substantially opened, when the engine speed increases, the rod 50 moves rightwardly in the drawing by the operation of the centrifugal governor 47 so that when the engine speed reaches n the rod 50 contacts with the lever 57 at one end and accordingly the other end of the lever 57 is rocked around the roll support 46 at the center of the lever 57 to cause the other end of the lever 57 to move leftwardly in the drawing against the tension of the spring 58 and accordingly the auxiliary throttle valve is gradually opened through the rod 56 connected with the other end of the lever 57 and the lever 54 connected at one end with the other end of the rod 55 and interlocked at the other end with the throttle valve 25. In this case, the tension of the spring 53 in the centrifugal governor 47 is so determined as to fully open the auxiliary throttle valve 25 when the engine speed reaches n and the stop 59 is so disposed as to stop the lever 57 at one end when the engine speed exceeds the speed n Thus, the area designated by in graph shown in FIG. 3 is improved by the opening operation of the auxiliary throttle valve 25. The opening of the auxiliary throttle valve 25 is shown in the graph illustrated in FIG. 9.

Referring now to FIG. 10, which shows a control device for electrically opening and closing the auxiliary throttle valve in accordance with the present invention, the electrical control device 60 comprises an accelerator switch 61 connected with the accelerator pedal 18 interlocking with the main throttle valve 15 to operate to open or close its contact in response to the opening of the main throttle valve 15 so that when the main throttle valve 15 fully opens or substantially opens the contact of the switch 61 closes while when the main throttle valve 15 does not fully open nor substantially open the contact of the switch 61 opens. The output of a generator 62 for generating a voltage proportional to the engine speed to detect the engine speed is connected to a voltage control device 63 for energizing a relay 64. When the engine speed reaches a valve n so that the generator 62 generates a voltage V corresponding to the engine speed n, the contact of the relay 64 closes by the energization of the voltage control device 63. A solenoid valve 65 connected with the auxiliary throttle valve 25 is energized from an electric power source 66 when the main throttle valve 15 fully opens or substantially opens, and the engine speed has reached a valve n so that the relay 64 is closed.

In operation of the device thus constructed, when the main throttle valve 15 fully opens or substantially opens by the depression of the accelerator pedal 18, the contact of the switch 61 is closed and when the engine speed reaches the speed n so that the relay 64 is energized by the voltage generated at the generator 62 through the voltage control device 63 to cause the contact of the relay 64 to be closed with the result that the solenoid valve 65 is energized from the source 66 through the contact of the switch 61 and the contact of the relay 64 and accordingly the auxiliary throttle valve 25 is abruptly opened as shown in the graph in FIG. 12. Thus, the area designated by 32 in graph shown in FIG. 4 is improved thereby in the opening operation of the auxiliary throttle valve Referring now to FIG. 11, the electrical control device 60 may comprise the accelerator switch 61 connected with the accelerator pedal 18 interlocking with the main throttle valve 15 to operate to open or close its control its contact in response to the opening of the main throttle valve 15, a relay 64 having a relay coil 64a electrically connected between the accelerator switch 61 and the ground and a contact 64b to be contacted when the relay coil 64a is energized by the ac celerator switch 61 the solenoid valve 65 electrically connected with the contact 64b of the relay 64 and to be energized to let the auxiliary throttle valve 25 to fully open when the relay 64 is energized by the accelerator switch 61 wherein when the main throttle valve 15 is fully opened the accelerator switch 61 becomes ON.

In thethus constructed control device 61, when the main throttle valve 15 fully opens, the auxiliary throttle valve 25 is always opened with the result that the torque curve designated by a broken line in FIG. 2 is obtained. It means that while the main throttle valve 15 fullyopens the auxiliary throttle valve 25 fully opens in all range of the engine speed.

The main throttle valve 15 may be directly connected mechanically with the auxiliary throttle valve 25 wherein the amount of the intaken air is controlled by both the main and auxiliary throttle valves 15 and 25 so that both the valves 15 and 25 may be fully opened. Thus, the torque curve designated by the broken line in FIG. 2 may also be obtained.

I claim:

1. An intake manifold of a fuel injection system of an internal combustion engine comprising a main throttle valve chamber open to the atmosphere, a main throttle valve mounted in said main throttle valve chamber and operated by an accelerator pedal, an air chamber communicating with said main throttle valve chamber downstream of said throttle valve, a plurality of branches leading from said air chamber to respective cylinders of the engine, an auxiliary throttle valve chamber open to the atmosphere and communicating with said air chamber at a position spaced from said main throttle valve chamber, an auxiliary throttle valve mounted in said auxiliary throttle valve chamber, means responsive to the speed of the engine and auxiliary-throttle-valve-actuating means connected with said main throttle valve and with said speed responsive means for opening said auxiliary throttle valve only when said main throttle valve is open not less than a selected amount and when the engine speed exceeds a selected value.

2. An intake manifold according to claim 1, wherein the value of engine speed above which said auxiliary throttle valve is opened by said actuating means is the value at which the engine torque with said auxiliary valve closed reaches a maximum value.

3. An intake manifold according to claim 1, wherein said actuating means operates to open said auxiliary throttle valve fully when said main throttle valve is open not less than said selected amount and the engine speed reaches said selected value.

4. An intake manifold according to claim 1, wherein said actuating means operates to open said auxiliary throttle valve progressively when said main throttle valve is open not less than a selected amount and the engine speed goes above said selected value.

5. An intake manifold according to claim 1, wherein one said throttle valve chamber communicates with said air chamber at one end thereof and the other said throttle valve chamber communicates with said air chamber intermediate its ends.

6. An intake manifold as set forth in claim 1, wherein said air chamber is connected with the main throttle valve chamber at one end thereof and said auxiliary throttle valve chamber is connected with the other end thereof.

7. An intake manifold as set forth in claim 1, wherein said branches are formed straight whereby the intake air tends to be accelerated by the intake vacuum in the cylinder and then tends to enter the cylinder abruptly.

8. An intake manifold as set forth in claim 1, wherein said actuating means is a control device for mechanically opening and closing the auxiliary throttle valve and including an angled lever pivotally connected to the vehicle body and having two arms one of which is connected with said main throttle valve to cause said lever to be interconnected with the main throttle valve, a first lever pivotally connected with the vehicle body at one end and connected at an intermediate point with the other arm of said angled lever to cause the first lever to be interconnected with the angled lever and a roller mounted at the other end of said first lever, a centrifugal governor driven by the engine and having a plurality of weights, a plurality of angled levers each connected at one end with the weight, a first rod having a disc fixed at one end thereof and engaged at one surface with the other ends of the angled levers and inserted at the other through a first hole formed in the vehicle body, a spring so disposed between the disc of the rod and the vehicle body as to press the disc against the centrifugal force of the weights applied to the other surface of the disc, a second lever supported at an intermediate point by the vehicle body and interconnected with the auxiliary throttle valve at one end, a second rod slidably inserted into a second hole formed in the vehicle body and connected at one end with the other end of the second lever, a third lever pivotally supported at an intermediate portion by said roller on the first lever and connected at one end with the other end of the second rod and with a second spring which is fixed at one end to the vehicle body for biasing the second lever and engaged with the end of the first rod at the other end, said intermediate portion of said third lever being so formed with a recess as to move the auxiliary throttle valve in response to the movement of the main throttle valve and engine speed, and a stop for limiting the movement of the third lever.

9. An air intake manifold as set forth in claim 1, wherein said actuating means comprises a control device for electrically opening and closing the auxiliary throttle valve and including an accelerator switch connected with the accelerator pedal interconnected with the main throttle valve to operate to open or close its contact in response to the opening of the main throttle valve so that when the main throttle valve fully opens or substantially opens the contact of the switch closes, a generator for generating a voltage proportional to the engine speed, a control relay, a voltage control device for energizing said relay when the engine speed reaches a predetermined speed so that the generator generates a voltage corresponding to the predetermined engine speed and accordingly the contact of the relay closes by the energization of the voltage control device, a solenoid connected with the auxiliary throttle valve and energized when the main throttle valve fully opens or substantially opens, and an electric power source for energizing the solenoid through said switch and relay. 

1. An intake manifold of a fuel injection system of an internal combustion engine comprising a main throttle valve chamber open to the atmosphere, a main throttle valve mounted in said main throttle valve chamber and operated by an accelerator pedal, an air chamber communicating with said main throttle valve chamber downstream of said throttle valve, a plurality of branches leading from said air chamber to respective cylinders of the engine, an auxiliary throttle valve chamber open to the atmosphere and communicating with said air chamber at a position spaced from said main throttle valve chamber, an auxiliary throttle valve mounted in said auxiliary throttle valve chamber, means responsive to the speed of the engine and auxiliarythrottle-valve-actuating means connected with said main throttle valve and with said speed responsive means for opening said auxiliAry throttle valve only when said main throttle valve is open not less than a selected amount and when the engine speed exceeds a selected value.
 2. An intake manifold according to claim 1, wherein the value of engine speed above which said auxiliary throttle valve is opened by said actuating means is the value at which the engine torque with said auxiliary valve closed reaches a maximum value.
 3. An intake manifold according to claim 1, wherein said actuating means operates to open said auxiliary throttle valve fully when said main throttle valve is open not less than said selected amount and the engine speed reaches said selected value.
 4. An intake manifold according to claim 1, wherein said actuating means operates to open said auxiliary throttle valve progressively when said main throttle valve is open not less than a selected amount and the engine speed goes above said selected value.
 5. An intake manifold according to claim 1, wherein one said throttle valve chamber communicates with said air chamber at one end thereof and the other said throttle valve chamber communicates with said air chamber intermediate its ends.
 6. An intake manifold as set forth in claim 1, wherein said air chamber is connected with the main throttle valve chamber at one end thereof and said auxiliary throttle valve chamber is connected with the other end thereof.
 7. An intake manifold as set forth in claim 1, wherein said branches are formed straight whereby the intake air tends to be accelerated by the intake vacuum in the cylinder and then tends to enter the cylinder abruptly.
 8. An intake manifold as set forth in claim 1, wherein said actuating means is a control device for mechanically opening and closing the auxiliary throttle valve and including an angled lever pivotally connected to the vehicle body and having two arms one of which is connected with said main throttle valve to cause said lever to be interconnected with the main throttle valve, a first lever pivotally connected with the vehicle body at one end and connected at an intermediate point with the other arm of said angled lever to cause the first lever to be interconnected with the angled lever and a roller mounted at the other end of said first lever, a centrifugal governor driven by the engine and having a plurality of weights, a plurality of angled levers each connected at one end with the weight, a first rod having a disc fixed at one end thereof and engaged at one surface with the other ends of the angled levers and inserted at the other through a first hole formed in the vehicle body, a spring so disposed between the disc of the rod and the vehicle body as to press the disc against the centrifugal force of the weights applied to the other surface of the disc, a second lever supported at an intermediate point by the vehicle body and interconnected with the auxiliary throttle valve at one end, a second rod slidably inserted into a second hole formed in the vehicle body and connected at one end with the other end of the second lever, a third lever pivotally supported at an intermediate portion by said roller on the first lever and connected at one end with the other end of the second rod and with a second spring which is fixed at one end to the vehicle body for biasing the second lever and engaged with the end of the first rod at the other end, said intermediate portion of said third lever being so formed with a recess as to move the auxiliary throttle valve in response to the movement of the main throttle valve and engine speed, and a stop for limiting the movement of the third lever.
 9. An air intake manifold as set forth in claim 1, wherein said actuating means comprises a control device for electrically opening and closing the auxiliary throttle valve and including an accelerator switch connected with the accelerator pedal interconnected with the main throttle valve to operate to open or close its contact in response to the opening of the main throttle valve so that when the maIn throttle valve fully opens or substantially opens the contact of the switch closes, a generator for generating a voltage proportional to the engine speed, a control relay, a voltage control device for energizing said relay when the engine speed reaches a predetermined speed so that the generator generates a voltage corresponding to the predetermined engine speed and accordingly the contact of the relay closes by the energization of the voltage control device, a solenoid connected with the auxiliary throttle valve and energized when the main throttle valve fully opens or substantially opens, and an electric power source for energizing the solenoid through said switch and relay. 